We recently moved into a new house with existing rose plants, many of which are diseased. The obvious problems are the orange spots that puff into dust when you touch the plant, and black / brown patches on the leaves.

I tried a compound from Bayer that was said to resolve all problems with roses including the ones I described, but it had no effect. It was pellets that you put onto the soil.

I'm wondering if it's worth trying to save these plants, or if I should simply pull them out and plant new ones.

I have some Daconil fungicide on hand, which I understand can be used to treat some things. That would be my next attempt.

EDIT: Some of the plants have a whitish color on the leaves / stems, wbich I understand is also fungus related.

1 Answer
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It sounds like you have powdery mildew (whitish film on stems and leaves) and rose rot (orange spots) and blackspot, "Diplocarpon rosae". That is quite a handful of problems for a rose but there is nothing here that good garden practices, soil amendment and some control measures like sulphur can't handle.

Here are my suggestions:

remove all debris from around the roses. Spores will find a nice home here.

prune hard to encourage air circulation and sun, remove or prune other shrubs and trees if necessary

make sure that the soil is not waterlogged due to poor drainage or grading. Topdress or regrade if necessary

use a mulch such as bark, shredded cedar, wood chips

cut all infected parts off, with a bad infestation consider an industrial solution and cut the roses down to the ground. (However, if they are grafted, cut everything more then six inches above the ground, which is hopefully above the graft). Remove infected material, do not compost this.

when new spots reoccur apply sulphur, soap and water as a spray after every rain

do not use fertilizers this spring as they promote growth which could be more attractive to fungal and viral diseases. If there is no sign of problems next spring then consider fertilizing

Some rose varieties are prone to black spot and if these are members of those varieties you may wish to consider removing them and planting new resistant types such as the ones listed here

Edit: With the scale of the problem you have I would act immediately. Waiting will only allow the problems to increase.